Apparatus and method to provide a local oscillator signal

ABSTRACT

An apparatus provides a local oscillator signal based on a selected channel of an RF input signal. For example, the apparatus can set a frequency of the local oscillator signal based on the selected channel. Digital circuitry can be used to generate the local oscillator signal. For instance, the digital circuitry can provide a digital representation of the local oscillator signal. A DAC can convert the digital representation to an analog signal. Other circuitry can provide first and second quadrature components of the local oscillator signal, based on the analog signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to local oscillator circuitry,and more specifically to providing a local oscillator signal usingdigital circuitry.

2. Background Art

Television signals are transmitted at radio frequencies (RF) usingterrestrial, cable, or satellite transmission schemes. Terrestrial andcable TV signals are typically transmitted at frequencies ofapproximately 57 to 860 MHZ, with 6 MHZ channel spacing in the UnitedStates and 8 MHz channel spacing in Europe. Satellite TV signals aretypically transmitted at frequencies of approximately 980 to 2180 MHz.

Regardless of the transmission scheme, a tuner is utilized to select anddown-convert a desired channel from the TV signal to an intermediatefrequency (IF) signal or a baseband signal, which is suitable forprocessing and display on a TV or computer screen. The tuner shouldprovide sufficient image rejection and channel selection duringdown-conversion as is necessary for the specific application. TheNational Television Standards Committee (NTSC) sets standards fortelevision signal transmission, reception, and display. To process aNTSC signal, it is preferable that the tuner have a high-level of imagerejection. However, less image rejection is acceptable for non-NTSCsignals depending on the specific application and the correspondingdisplay requirements.

A downconverter generally mixes a local oscillator signal and the RFinput signal to provide the downconverted signal. The circuitry used togenerate the local oscillator signal often includes large inductorsand/or capacitors, which can increase the size and cost of thecircuitry.

What is needed is a method or apparatus for providing the localoscillator signal using digital circuitry.

BRIEF SUMMARY OF THE INVENTION

The present invention is an apparatus and method for providing a localoscillator signal using digital circuitry. For example, the apparatuscan include digital circuitry to facilitate generation of the localoscillator signal. According to an embodiment, an apparatus includes alocal oscillator circuit, a digital-to-analog converter (DAC), andanother circuit. The local oscillator circuit provides a digitalrepresentation of the local oscillator signal. The DAC converts thedigital representation to an analog signal. The other circuit providesfirst and second quadrature components of the local oscillator signal,based on the analog signal. For instance, the other circuit can includea phase locked loop (PLL) and an oscillator. The PLL generates the localoscillator signal based on the analog signal. The oscillator providesthe first and second quadrature components of the local oscillatorsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates a communication system according to an embodiment ofthe present invention;

FIG. 2 illustrates channel selection of the receiver according to anembodiment of the present invention;

FIG. 3 illustrates a receiver in which quadrature components of a localoscillator signal are independently generated according to an embodimentof the present invention;

FIG. 4 illustrates a receiver in which quadrature components of a localoscillator signal are generated using an oscillator according to anembodiment of the present invention;

FIG. 5 illustrates a receiver in which quadrature components of a localoscillator signal are generated using dividers according to anembodiment of the present invention;

FIG. 6 illustrates a receiver in which quadrature components of a localoscillator signal are generated using a filter according to anembodiment of the present invention;

FIG. 7 illustrates a receiver in which mixers are not needed in thedigital domain according to an embodiment of the present invention;

FIG. 8 illustrates a receiver having multiple analog-to-digitalconverters (ADCs) according to an embodiment of the present invention;

FIG. 9 illustrates a receiver having a baseband equalizer according toan embodiment of the present invention;

FIG. 10 illustrates a flow chart of a method of processing a RF inputsignal having a plurality of channels according to an embodiment of thepresent invention;

FIG. 11 illustrates a more detailed flow chart of a method of processinga RF input signal having a plurality of channels according to anembodiment of the present invention;

FIG. 12 illustrates a flow chart of a method of setting a frequency of alocal oscillator signal according to an embodiment of the presentinvention; and

FIG. 13 illustrates a flow chart of another method of setting afrequency of a local oscillator signal according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a communication system according to an embodiment ofthe present invention. The communication system 100 includes a channelselector 110, a receiver 120, and a display device 130. For example, auser selects a desired channel using the channel selector 110. Thechannel selector 110 transmits a signal 104 including informationassociated with the desired channel to the receiver 120. In anembodiment, the channel selector 110 transmits an infrared signalindicating the desired channel to the receiver 120. It is understood inthe art that the channel selector 110 is physically coupled to thereceiver 120 in some embodiments.

The receiver 120 receives a radio frequency (RF) input signal 102 andthe signal 104 from the channel selector 110. The RF input signal 102typically includes multiple channels. The receiver 120 uses the signal104 from the channel selector 110 to determine which of the channels ofthe RF input signal 102 to transmit to the display device 130.

The display device 130 can be a cathode ray tube (CRT) display device, aliquid crystal display (LCD) device, a plasma display device, or animage projection device, to provide some examples. The display device130 provides a pictorial representation of the selected channel. In anembodiment, the display device 130 is capable of accepting a signalhaving a higher resolution than a standard National Television StandardsCommittee (NTSC) signal. For example, the display device 130 can becapable of accepting an enhanced-definition television (EVTV) signal ora high-definition television (HDTV) signal.

The operation of the receiver 120 is described as follows and inreference to FIG. 2, where FIG. 2 represents the frequency spectrum ofthe particular signals that are received and/or generated by thereceiver 120. As shown in FIG. 2, in one embodiment, the RF input signal102 can include channels within a frequency range from 57 to 860 MHz.For example, the RF input signal 102 can be a cable television signalhaving a channel spacing of 6 or 8 MHz, although the scope of thepresent invention is not limited in this respect. As shown in FIG. 2, achannel at 585 MHz can be selected as an example. Upon selection of thechannel, the RF input signal 102 is generally downconverted to providean IF signal or a baseband signal. For example, the 585 MHz channel canbe downconverted to facilitate processing of the channel prior to itsdisplay. As shown in FIG. 2, the 585 MHz channel can be downconverted to3 or 4 MHz in an embodiment.

The downconverted signal is typically generated by combining a localoscillator signal and the selected channel of the RF input signal 102 ina receiver 120. For example, a mixer can provide a downconverted signalhaving a frequency based on the frequency of the local oscillatorsignal. In a first embodiment, the local oscillator signal is aquadrature signal having first and second LO quadrature components.Generally, quadrature components are substantially the same in amplitudeand frequency; however, the two components are typically 90° out ofphase with each other. In a second embodiment, the RF input signal is aquadrature signal having first and second RF quadrature components. In athird embodiment, the local oscillator signal and the RF input signaleach have quadrature components.

With respect to the third embodiment, each of the RF quadraturecomponents can be combined with each of the LO quadrature components.The LO quadrature components can be referred to as LO_(i) and LO_(q).The RF quadrature components can be referred to as RF_(i) and RF_(q).For example, mixing the RF quadrature components and the LO quadraturecomponents can provide quadrature signals defined by the followingequations:IF _(i) =LO _(i) *RF _(i) +LO _(q) *RF _(q)IF _(q) =LO _(i) *RF _(q) −LO _(q) *RF _(i).

To simplify the discussion, the first embodiment is described withreference to FIGS. 3-5, which provide some examples of receivers thatutilize a local oscillator having first and second quadrature componentsto provide the downconverted signal, according to embodiments of thepresent invention.

FIG. 3 illustrates a receiver in which quadrature components of a localoscillator signal are independently generated according to an embodimentof the present invention. The receiver 300 includes a direct downconversion circuit 310, a demodulation circuit 360, and a localoscillator circuit 370. The direct down conversion circuit 310 receivesa RF input signal 102. The RF input signal 102 is generally amplified bya low-noise amplifier 312 to amplify the RF input signal 102 to anamplitude above the noise floor of the receiver 300. According to anembodiment, the RF input signal 102 is amplified before being receivedby the direct down conversion circuit 310. For instance, a discretelow-noise amplifier, such as Broadcom part number BCM 3405, can becoupled to the input of the direct down conversion circuit 310. In anembodiment, the RF input signal 102 is amplified by the direct downconversion circuit 310. For example, low-noise amplifiers 312 canamplify the RF input signal 102 before the RF input signal 102 is passedto mixers 314.

The mixers 314 mix the RF input signal 102 and a local oscillator signalto provide a downconverted signal. As shown in FIG. 3, mixer 314 a mixesthe RF input signal 102 and a first quadrature component of the localoscillator signal to provide a first downconverted quadrature component.Mixer 314 b mixes the RF input signal 102 and a second quadraturecomponent of the local oscillator signal to provide a seconddownconverted quadrature component. For instance, the downconvertedquadrature components can include unwanted adjacent channel energy. Oneor more low pass filters (LPFs) 316 can eliminate or reduce the unwantedenergy.

A multiplexer 320 can be included to select the first downconvertedquadrature component or the second downconverted quadrature component tobe sent to at least one analog-to-digital converter (ADC) 330. In FIG.3, the receiver 300 includes a single ADC 330 for illustrative purposes,though the scope of the present invention is not limited in thisrespect. For instance, a single ADC can be used to reduce gain and/orlinearity mismatches between the quadrature components. In anembodiment, using a single ADC reduces the size of the receiver 300.

The multiplexer 320 can interleave samples of the first downconvertedquadrature component and the second downconverted quadrature componentto provide an interleaved sample of the downconverted quadraturecomponents to the ADC 330. In one embodiment, the multiplexer 320toggles at a rate equal to at least twice the effective sampling rate ofthe ADC 330. For example, sampling at this rate can facilitate accurateconversion of the downconverted quadrature components by the ADC 330.

The ADC 330 converts the interleaved sampling of the downconvertedquadrature components into a digital signal. According to an embodiment,the sampling rate of the ADC 330 equals the interleaving rate of themultiplexer 320 plus an over sampling ratio. For instance, basing thesampling rate of the ADC 330 on the over sampling ratio can extend thenoise performance of the ADC 330 and/or reduce the number of bitsrequired by the ADC 330.

A demultiplexer 340 de-interleaves the digital samples of thedownconverted quadrature components provided by the ADC 330. In anembodiment, the demultiplexer 340 toggles at a rate equal to the togglerate of the multiplexer 320. The de-interleaved samples of thedownconverted quadrature components can be frequency shifted or timeshifted to restore quadrature alignment and/or quadrature timealignment, although the scope of the present invention is not limited inthis respect. For example, mixers 350 can introduce a frequency offsetto the de-interleaved samples of at least one of the downconvertedquadrature components to provide frequency-corrected samples to thedemodulation circuit 360.

The demodulation circuit 360 provides a demodulated signal to the localoscillator circuit 370. In an embodiment, the demodulation circuit 360is a quadrature amplitude modulation (QAM) demodulation circuit. Forexample, the demodulation circuit 360 can include a Nyquist filter, avariable rate symbol demodulator, an equalizer, and a carrier recoveryloop. According to an embodiment, QAM improves the data transmissionrate of the receiver 300 without degrading the bit error rate (BER) ofthe receiver 300.

The local oscillator circuit 370 sets the frequency of the localoscillator signal based on the selected channel of the RF input signal102. For example, the local oscillator circuit 370 can receiveinformation regarding the desired channel from the channel selector 110shown in FIG. 1 and set the frequency of the local oscillator signalbased on that information.

Receivers typically include at least one voltage controlled oscillator(VCO) that generates a signal having a frequency based on the inputvoltage of the VCO. According to an embodiment, the local oscillatorcircuit 370 includes a VCO. For example, each of the channels of the RFinput signal can be associated with a particular LO frequency needed todownconvert the selected channel. The VCO can receive an input voltagebased on the desired channel and set the frequency of the localoscillator signal based on the input voltage.

The local oscillator circuit 370 digitally generates the localoscillator signal according to an embodiment. For instance, the localoscillator circuit 370 typically generates a digital representation ofthe local oscillator signal. The receiver 300 often includes a memory372 to store a read-only memory (ROM) lookup table. The ROM lookup tablecan include a plurality of entries. According to a first embodiment,each entry represents a phase of the local oscillator signal or a sineor cosine thereof. The local oscillator circuit 370 can retrieve anentry from the ROM lookup table at each cycle or half-cycle of the VCOclock, for example, to provide the digital representation of the localoscillator signal.

According to another embodiment, the ROM lookup table stores an offsetvalue. For example, the offset value can indicate a difference betweenthe actual frequency of the local oscillator signal and the desiredfrequency of the local oscillator signal. The frequency of the localoscillator signal can be set based on the offset value. For instance,the offset value can be combined with the local oscillator signal toprovide a frequency-shifted local oscillator signal.

In another example, the offset value can indicate a difference betweenthe actual phase of the local oscillator signal and the desired phase ofthe local oscillator signal. The phase of the local oscillator signalcan be set based on the offset value. For instance, the offset value canbe combined with the local oscillator signal to provide a phase-shiftedlocal oscillator signal. Basing the frequency or the phase of the localoscillator signal on the offset value can save time, as compared toaccessing the ROM lookup table in successive cycles of the localoscillator circuit 370.

Digitally generating the local oscillator signal can enable a reductionin the number of VCOs needed in the receiver 300. For instance, areduction in the number of VCOs can provide a reduction in the size ofthe receiver 300. Including fewer VCOs in the receiver 300 can result ina lower cost of the receiver 300.

According to an embodiment, the local oscillator circuit 370 is a directdigital frequency synthesizer (DDFS). The DDFS digitally converts phaseinformation relating to the local oscillator signal to a digitizedsinusoidal waveform. The DDFS can receive the phase information from theROM lookup table or from the demodulated signal received from thedemodulation circuit 360, to provide some examples. The DDFS can providefaster frequency switching, lower phase noise, and/or higher frequencyresolution, as compared to standard phase-locked loop (PLL) frequencysynthesizers.

The DDFS typically includes a phase accumulator 374 to receive phaseinformation relating to the local oscillator signal with each successiveclock cycle of the local oscillator circuit 370. For example, the phaseaccumulator 374 can receive first phase information during a first clockcycle, second phase information during a second clock cycle, and so on.

The DDFS can further include a phase-to-sine converter 376 to convertphase information received from the memory 372 into a digitizedsinusoidal waveform. For example, the phase-to-sine converter 376 canprovide a first waveform representing the sine of the phase informationand a second waveform representing the cosine of the phase information.In an embodiment, the first waveform is a first quadrature component ofthe local oscillator signal, and the second waveform is a secondquadrature component of the local oscillator signal.

The memory 372 typically stores information relating to time-independentvariations between the quadrature components of the local oscillatorsignal. The DDFS generally monitors time-dependent variations betweenthe quadrature components. For instance, the DDFS can monitor thequadrature components of the local oscillator signal in the analogdomain. This can reduce the size and/or number of components needed inthe receiver 300.

Quadrature components of the local oscillator signal can be generatedindependently in accordance with the embodiment shown in FIG. 3.According to an embodiment, the local oscillator circuit 370 reduces again mismatch or a phase mismatch between the quadrature components. Forexample, the local oscillator circuit 370 can access the ROM lookuptable to determine a phase offset or a frequency offset to be applied toone of the quadrature components.

The offset value stored in the ROM lookup table can indicate a phasedifference between quadrature components of the local oscillator signal,for example. The offset value can be used to adjust the phase of atleast one of the quadrature components of the local oscillator signal.Utilizing the offset value to correct the phase difference between thequadrature components of the local oscillator signal can eliminate theneed for other quadrature correcting circuitry or software. For example,correcting the quadrature of the local oscillator signal using the localoscillator circuit 370 can reduce the number of components needed in thereceiver 300, thereby reducing the cost of the receiver in anembodiment.

The frequency of the local oscillator signal can be based on a frequencycontrol word associated with the local oscillator signal. For instance,a clock signal can be multiplied by the frequency control word tocalculate the frequency of the local oscillator signal. The offset valuestored in the ROM lookup table can be used to calculate the frequencycontrol word associated with the local oscillator signal. In anembodiment, the offset value is used to set the frequency of at leastone of the quadrature components of the local oscillator signal.

According to an embodiment, the receiver 300 includes two DDFSs. Forinstance, a first DDFS can be used to convert phase information relatingto a first quadrature component of the local oscillator signal to afirst digitized sinusoidal waveform. The second DDFS can be used toconvert phase information relating to a second quadrature component ofthe local oscillator signal to a second digitized sinusoidal waveform.

As shown in FIG. 3, digital representations of the local oscillatorquadrature components are provided to digital-to-analog converters(DACs) 380. The DACs 380 can convert the digital representations intoanalog local oscillator signals. For instance, the DACs 380 can directlygenerate the analog local oscillator signals. Alternatively, the DACs380 can generate reference signals, which can be used by phase-lockedloops (PLLs), such as PLLs 392, to generate the analog local oscillatorsignals. According to an embodiment, the DACs 380 reduce jitter of theanalog local oscillator signals.

Passing the local oscillator signal through a filter 390 can eliminateor reduce energy at frequencies outside the passband of the filter 390.The filter 390 can be a low pass filter or a bandpass filter, to providesome examples. According to an embodiment, the filter 390 is anarrow-band filter. In FIG. 3, the filter 390 can be set at a particularfrequency or range of frequencies that represents the desired channel ofthe RF input signal. In a first embodiment, the passband of the filter390 is set based on the frequency of the local oscillator signal set bythe local oscillator circuit 370. In a second embodiment, the passbandof the filter 390 is set at a predetermined frequency or range offrequencies, and the local oscillator circuit 370 manipulates thefrequency of the local oscillator signal to be within the passband ofthe filter 390. For example, the local oscillator circuit 370 canmultiply the frequency of the local oscillator signal by a factor basedon the selected channel of the RF input signal 102.

The filter 390 generally includes at least one phase-locked loop (PLL)392. The PLLs 392 can provide the quadrature components of the localoscillator signal to the direct down conversion circuit 310 to be mixedwith the RF input signal 102. As shown in FIG. 3, a PLL 392 can beincluded for each quadrature component of the local oscillator signal.However, a single PLL can be used to filter both quadrature components.

The PLL 392 often manipulates the frequency of the local oscillatorsignal by a predetermined factor. According to an embodiment, the PLL392 multiplies the frequency of the local oscillator signal by a factorin a range from approximately two to approximately thirty. The PLL 392can increase the frequency of the local oscillator signal by a factor ofsix in a cable modem system, for example. The PLL 392 can increase thefrequency of the local oscillator signal by a factor of twelve in asatellite communication system, to provide another example.

Using the PLL 392 to multiply the frequency of the local oscillatorsignal by a fixed value can allow the DAC 380 to sample at a lower rate.For example, the sampling rate of the DAC 380 is decreased by a factorthat is proportional to the factor by which the frequency of the localoscillator is multiplied in an embodiment. Using the PLL 392 to multiplythe frequency of the local oscillator signal by a fixed value can enablethe size of the PLL 392 to be reduced, as compared to the situation inwhich the PLL 392 is used to multiply the frequency of the localoscillator by a variable factor to generate the frequency of the localoscillator signal.

According to an embodiment of the present invention, the direct downconversion circuit 310, the demodulation circuit 360, and the localoscillator circuit 370 are on a common substrate. One or more of themultiplexer 320, the ADC 330, the demultiplexer 340, the DAC 380, andthe filter 390 can be on the common substrate, as well. Combiningelements, such as those mentioned above, on a common substrate canreduce the cumulative circuit area required by the elements. Reducingthe circuit area reduces the cost of the elements in an embodiment.

FIG. 4 illustrates a receiver in which quadrature components of a localoscillator signal are generated using an oscillator according to anembodiment of the present invention. For example, the local oscillatorcircuit 370 can provide a digital representation of a local oscillatorsignal that does not include quadrature components. The digitalrepresentation can be received by a single DAC 410, as shown in FIG. 4,though the scope of the invention is not limited in this respect. TheDAC 410 converts the digital representation of the local oscillatorsignal to an analog reference signal. The reference signal oftenincludes quantization noise and/or images, which can be related to thefinite sampling rate of the DAC 410, for example. The PLL 420 can filterthe reference signal to reduce or eliminate the quantization noiseand/or images. The PLL 420 generates the local oscillator signal basedon the reference signal. The oscillator 430 generates quadraturecomponents of the local oscillator signal to be provided to the directdown conversion circuit 310. According to an embodiment, the PLL 420includes the oscillator 430. For instance, the oscillator 430 can beembedded in the PLL 420.

The receiver 400 can include feedback 440 between the oscillator 430 andthe PLL 420. For example, the PLL 420 can use information received fromthe oscillator 430 via the feedback 440 to increase the frequency of thelocal oscillator signal. According to an embodiment, the PLL 420generates the local oscillator signal having a frequency that is basedon the reference signal and the information received from the oscillator430 via the feedback 440.

The oscillator 430 can be one or more ring oscillators orinductor-capacitor (LC) oscillators, to provide some examples. A ringoscillator generally has a greater bandwidth than a single LC oscillatorand requires less circuit area than multiple LC oscillators. A ringoscillator typically includes a plurality of inverters. For example, thering oscillator can include n inverters. Each inverter can have an inputand an output. The inverters can be coupled, such that the output of afirst inverter is coupled to the input of a second inverter, and theoutput of the second inverter is coupled to the input of a thirdinverter, etc. For instance, the output of the nth inverter can becoupled to the input of the first inverter. In an embodiment, the firstmixer 314 a of the direct down conversion circuit 310 is coupled to aparticular inverter. The second mixer 314 b can be coupled to anotherinverter to enable the signal received by the first mixer 314 a to be90° out of phase with the signal received by the second mixer 314 b.

An image filter can be coupled between the oscillator 430 and the directdown conversion circuit 310. For instance, the image filter can filterthe quadrature components of the local oscillator signal before passingthe quadrature components of the local oscillator signal to the mixers314. The DAC 410, the PLL 420, the oscillator 430, and/or the imagefilter can be disposed on a common substrate with the direct downconversion circuit 310, the demodulation circuit 360, and the localoscillator circuit 370.

FIG. 5 illustrates a receiver in which quadrature components of a localoscillator signal are generated using dividers according to anembodiment of the present invention. The oscillator 430 can oscillate ata frequency that is a multiple of the local oscillator (LO) signalfrequency. The dividers 510 can divide the frequency of the signalprovided by the oscillator 430 by a factor to provide the LO quadraturecomponents at a particular frequency.

For example, the oscillator 430 can oscillate at a frequency twelvetimes the LO signal frequency. The dividers 510 can be divide-by-twodividers. In this example, the divide-by-two dividers can divide thefrequency of the signal that is provided by the oscillator 430 by two toprovide quadrature components having a frequency of six times the LOsignal frequency.

The dividers 510 can be initialized one-half of an input cycle apart,for example. The dividers 510 can be triggered on alternating edges ofthe signal provided by the oscillator 430. The first divider 510 a canbe triggered on a rising edge of the signal provided by the oscillator430, and the second divider 510 can be triggered on a falling edge ofthe signal, or vice versa. The resulting LO quadrature components aretypically 90° out of phase with each other. The oscillator 430 can be adifferential oscillator to provide a signal having symmetrical risingand falling edges.

According to an embodiment, the dividers 510 are coupled between the PLL420 and the oscillator 430. For example, the dividers 510 can reduce thefrequency of the signal provided by the PLL 420 before passing thesignal to the oscillator 430. In an embodiment, a single divider iscoupled between the PLL 420 and the oscillator 430.

FIG. 6 illustrates a receiver in which quadrature components of a localoscillator signal are generated using a filter according to anembodiment of the present invention. The filter 610 generally removesjitter from the local oscillator signal. The filter 610 can be capableof accommodating a range of local oscillator frequencies.

The filter 610 can be an image filter. The filter 610 can be a low passfilter or a bandpass filter, to provide some examples. In an embodiment,the filter 610 is a poly-phase filter. The poly-phase filter generallyincludes a capacitor-resistor (CR) high pass filter portion and aresistor-capacitor (RC) low pass filter portion. LO quadraturecomponents can be provided respectively by the two filter portions. Atthe 3 dB point, for example, the magnitude of the LO quadraturecomponents is approximately the same, and phase of the two componentsdiffers by approximately 90°. The filter 610 can be adjustable toaccommodate particular local oscillator frequencies. For instance, thefrequency response of the filter 620 can be digitally programmed toaccommodate a range of local oscillator frequencies.

Referring to FIG. 7, mixers 350 as shown in FIGS. 3-6 are notnecessarily needed according to an embodiment of the present invention.For instance, the local oscillator circuit 370 can set the frequency ofthe local oscillator sufficiently, so that a frequency offset need notbe provided to the downconverted signal. In another example, the localoscillator circuit 370 and/or the ADC 330 reduce a gain mismatch or alinearity mismatch between quadrature components to a degree that mixersin the digital domain of the receiver 300, 400, 500, or 600 are notnecessary to adjust the frequency difference between quadraturecomponents.

Although the receivers 300, 400, 500, 600, and 700 of FIGS. 3-7,respectively, include a single ADC 330 for illustrative purposes, thescope of the present invention is not limited in this respect. Referringto FIG. 8, multiple ADCs 830 can be used to convert the downconvertedquadrature components into digital signals. For instance, a first ADC830 a can convert the first quadrature component into a first digitalsignal, and a second ADC 830 b can convert the second quadraturecomponent into a second digital signal. Using a different ADC 830 foreach downconverted quadrature component eliminates the need to have themultiplexer 320 and the demultiplexer 340, according to an embodiment.

FIG. 9 illustrates a receiver having a baseband equalizer according toan embodiment of the present invention. Quadrature paths of the receiver900 are generally not completely isolated from each other. For instance,a first quadrature component traveling along a first path 910 a caninclude information from a second quadrature component traveling along asecond path 910 b, and vice versa. The baseband equalizer 920 candetermine how much information from one quadrature component is includedin the other quadrature component, and vice versa. The basebandequalizer 920 generally subtracts the second quadrature componentinformation or a portion thereof from the first quadrature component.The baseband equalizer 920 typically subtracts the first quadraturecomponent information or a portion thereof from the second quadraturecomponent.

The baseband equalizer 920 can provide quadrature phase correction ofthe digitized downconverted signal. For instance, one of thedemultiplexed quadrature components received from the demultiplexer 340can be frequency shifted or phase shifted with respect to the otherdemultiplexed quadrature component. The baseband equalizer 920 canreduce or eliminate the difference in frequency and/or phase between thedemultiplexed quadrature components.

The baseband equalizer 920 generally includes a phase detector and anamplitude detector. The phase detector can detect a difference of phasebetween quadrature components. The amplitude detector can detect adifference of amplitude between the quadrature components.

FIG. 10 illustrates a flow chart of a method of processing a RF inputsignal 102 having a plurality of channels according to an embodiment ofthe present invention. A local oscillator circuit 370 can set thefrequency of a local oscillator at block 1010 based on a selectedchannel of the plurality of channels. For instance, the frequency of thelocal oscillator can be modified using the channel selector 110 as shownin FIG. 1. A direct down conversion circuit 310 can directly downconvertthe RF input signal 102 at block 1020 to provide a downconverted signalbased on the local oscillator signal. For example, at least one mixer314 can mix the local oscillator with the RF input signal 102 to providethe downconverted signal. In an embodiment, information not associatedwith the selected channel is removed from the downconverted signal. Forinstance, at least one low pass filter 316 can low pass filter thedownconverted signal to remove unwanted harmonics. A demodulationcircuit demodulates the downconverted signal at block 1030 to provide ademodulated signal.

Referring to FIG. 11, a low noise amplifier (LNA) amplifies the RF inputsignal 102 at block 1105 to ensure that its amplitude is above the noisefloor of the receiver 300, 400, or 500. Mixers 314 mix the RF inputsignal with local oscillator (LO) quadrature components at block 1110 toprovide downconverted quadrature components. For example, the directdownconversion circuit 310 can downconvert the RF quadrature componentsto an IF frequency or to baseband. At least one low pass filter (LPF)filters the downconverted quadrature components at block 1115 to removeunwanted harmonics. A multiplexer 320 can multiplex the downconvertedquadrature components at block 1120 at a multiplexing rate of at leasttwice the frequency of the downconverted quadrature components toprovide a multiplexed signal. An analog-to-digital converter (ADC) 330converts the multiplexed signal at block 1125 into a digital signal,which is demultiplexed at block 1130 by a demultiplexer 340 to providedigital quadrature components. In an embodiment, a demultiplexer 340demultiplexes the digital signal at the multiplexing rate.

Mixers 350 can combine a frequency offset with the digital quadraturecomponents at block 1135 to center the digital components in the Nyquistfilter bandwidth. A demodulation circuit 360 demodulates the digitalquadrature components at block 1140, so that they can be provided to asymbol mapper or a forward error correction (FEC) circuit, to providesome examples. A local oscillator circuit 370 can use the demodulatedquadrature components to set the frequency of the LO quadraturecomponents at block 1145. In an embodiment, setting the frequency of thelocal oscillator signal eliminates the need to combine the frequencyoffset with the digital quadrature components at block 1135.

Mismatches can occur between quadrature components. For example, thephase of one quadrature component can shift with respect to the otherquadrature component as the two components travel along their quadraturepaths. A mismatch, such as the phase mismatch just described, can becorrected by adjusting the phase difference between the LO quadraturecomponents at block 1150. For example, the local oscillator circuit 370can set the phase difference between LO quadrature components at a valuedifferent than 90° to take into consideration the mismatch. In anembodiment, setting the frequency of the LO quadrature components, asset forth at block 1145, includes adjusting the phase difference betweenthe LO quadrature components, as set forth at block 1150.

Mixers 350 correct imbalances between quadrature components of thedownconverted signal before the demodulator 360 demodulates thedownconverted signal, according to an embodiment. The local oscillatorsignal is generally based on the demodulated signal. For instance, thelocal oscillator signal can be based on a difference between quadraturecomponents of the downconverted signal.

At least one digital-to-analog converter (DAC) 380 converts the LOquadrature components to analog signals at block 1155. A filter 390 canfilter the LO quadrature components at block 1160 using at least onephase-locked loop (PLL) 392, for example. The local oscillator circuit370 generally performs operations using a digital representation of theLO signal, and the filter 390 typically performs operations using theanalog LO signal provided by the DAC 380. If a RF input signal isdetected, as determined at diamond 1165, processing the RF input signalcontinues with mixers 314 mixing the LO quadrature components and the RFinput signal, as set forth at block 1110. If no RF input signal isdetected, processing the RF input signal ends.

FIG. 12 illustrates a flow chart of a method of setting a frequency of alocal oscillator signal according to an embodiment of the presentinvention. The local oscillator circuit 370, for example, can read orsample the frequency, phase, or amplitude of the local oscillator signalto determine whether the phase or frequency of the local oscillatorsignal should be modified. The local oscillator circuit 370 can read orsample characteristics of the local oscillator signal using digitaland/or analog representations of the local oscillator signal. Forexample, the local oscillator circuit 370 can track time-dependenterrors in the receiver 300, 400, 500, 600, 700, 800, or 900.

A memory 372 can store the difference between the phase or frequency ofthe local oscillator signal and the desired phase or frequency. Thelocal oscillator circuit 370 can set the phase or frequency of the localoscillator based on the difference that is stored in memory 372. Thememory 372, for example, can store a read-only memory (ROM) lookup tableat block 1210. The ROM lookup table can include an offset value. Theoffset value can be based on a difference between the actual phase orfrequency of the local oscillator and a desired phase or frequency ofthe local oscillator signal. The local oscillator circuit 370, forexample, can compare the phase or frequency of the local oscillatorsignal to the desired phase or frequency to calculate the offset value.The local oscillator circuit 370 can retrieve the offset value from theROM lookup table at block 1220. At block 1230, the local oscillatorcircuit 370 sets the frequency of the local oscillator signal based onthe offset value.

Referring to FIG. 13, the ROM lookup table can include phase informationrelating to the local oscillator signal. For example, the phaseinformation can include a plurality of values, with each valuerepresenting a desired phase of the local oscillator signal for anassociated clock cycle. The local oscillator circuit 370 can retrievethe phase information at block 1320. In an embodiment, the localoscillator circuit 370 retrieves a value at each successive clock cycle.A particular value can be associated with more than one clock cycle. Thelocal oscillator signal is generated at block 1330 based on the phaseinformation. The sine or cosine of the values can be associated withsuccessive clock cycles, such that a discrete or digitized sinusoidalwaveform is provided in an embodiment. For instance, the localoscillator circuit 370 can generate a digital representation of thelocal oscillator signal, and the DAC(s) can convert the digitalrepresentation to the analog local oscillator signal.

In an embodiment, the local oscillator circuit 370 combines a phaseoffset with the value of the phase retrieved from the ROM lookup table.For example, manipulating the phase of the local oscillator signal canaccount for a phase shift that occurs during processing of the RF inputsignal. The phase of one quadrature component can shift more or lessthan the phase of the other quadrature component in some instances. Forinstance, differences in the quadrature paths can create a phase shiftbetween the quadrature components. The phase difference between LOquadrature components can be adjusted using the phase offset to accountfor this phase shift between quadrature components. In an embodiment,the local oscillator circuit 370 can use the phase offset to adjust thequadrature between the LO quadrature components to be a value other than90°.

CONCLUSION

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. An apparatus, comprising: a local oscillator circuit to provide adigital representation of a local oscillator signal; a digital-to-analogconverter (DAC) to convert the digital representation to an analogsignal; and a circuit to provide first and second quadrature componentsof the local oscillator signal, based on the analog signal.
 2. Theapparatus of claim 1, wherein the circuit includes: a phase locked loop(PLL) to generate the local oscillator signal based on the analogsignal; and an oscillator to provide the first and second quadraturecomponents of the local oscillator signal.
 3. The apparatus of claim 2,wherein the oscillator is a ring oscillator.
 4. The apparatus of claim2, wherein the PLL increases a frequency of the first and secondquadrature components by a factor.
 5. The apparatus of claim 1, whereinthe circuit is a filter.
 6. The apparatus of claim 1, further comprisinga divider to divide a frequency of the first and second quadraturecomponents of the local oscillator signal by a factor.
 7. The apparatusof claim 1, wherein the local oscillator circuit reduces a gainmismatch, a frequency mismatch, or a phase mismatch between the firstquadrature component and the second quadrature component.
 8. Theapparatus of claim 1, wherein the local oscillator circuit generatesfirst and second quadrature components of the digital representation. 9.The apparatus of claim 1, wherein the local oscillator circuit is adirect digital frequency synthesizer.
 10. The apparatus of claim 1,further comprising a memory to store phase information that is used toadjust at least one of the first and second quadrature components of thelocal oscillator signal.
 11. The apparatus of claim 1, furthercomprising a memory to store an offset value, wherein a phase or afrequency of at least one of the first and second quadrature componentsof the local oscillator signal is based on the offset value.
 12. Theapparatus of claim 1, further comprising a memory to store phaseinformation associated with the local oscillator signal, wherein thefirst quadrature component of the local oscillator signal is based on asine of the phase information, and wherein the second quadraturecomponent of the local oscillator signal is based on a cosine of thephase information.
 13. The apparatus of claim 1, further comprising amemory to store an offset value that is used to calculate a frequencycontrol word associated with at least one of the first and secondquadrature components of the local oscillator signal.
 14. The apparatusof claim 1, further comprising: a first direct digital frequencysynthesizer to provide a digital representation of the first quadraturecomponent of the local oscillator signal; and a second direct digitalfrequency synthesizer to provide a digital representation of the secondquadrature component of the local oscillator signal.
 15. A method,comprising: generating a digital representation of a local oscillatorsignal; converting the digital representation to an analog signal; andproviding first and second quadrature components of the local oscillatorsignal, based on the analog signal.
 16. The method of claim 15, furthercomprising dividing a frequency of the first and second quadraturecomponents of the local oscillator signal by a factor.
 17. The method ofclaim 15, further comprising reducing a gain mismatch, a frequencymismatch, or a phase mismatch between the first quadrature component andthe second quadrature component.
 18. The method of claim 15, whereinproviding the first and second quadrature components includes providingthe first and second quadrature components using a ring oscillator. 19.The method of claim 15, further including increasing a frequency of thefirst and second quadrature components by a factor, in response toconverting the digital representation to the analog signal.
 20. Themethod of claim 15, wherein generating the digital representation of thelocal oscillator signal includes generating first and second quadraturecomponents of the digital representation.
 21. The method of claim 15,wherein generating the digital representation of the local oscillatorsignal includes generating the digital representation of the localoscillator signal using a direct digital frequency synthesizer.
 22. Themethod of claim 15, further comprising storing phase information andadjusting at least one of the first and second quadrature components ofthe local oscillator signal based on the phase information.
 23. Themethod of claim 15, further comprising: storing an offset value; andbasing a phase or a frequency of at least one of the first and secondquadrature components of the local oscillator signal on the offsetvalue.
 24. The method of claim 15, further comprising: storing phaseinformation associated with the local oscillator signal; basing thefirst quadrature component of the local oscillator signal on a sine ofthe phase information; and basing the second quadrature component of thelocal oscillator signal on a cosine of the phase information.
 25. Themethod of claim 15, further comprising storing an offset value andcalculating a frequency control word associated with at least one of thefirst and second quadrature components of the local oscillator signal,based on the offset value.
 26. The method of claim 15, whereingenerating the digital representation of the local oscillator signalincludes generating a digital representation of the first quadraturecomponent of the local oscillator signal and a digital representation ofthe second quadrature component of the local oscillator signal.